1. Field of the Invention
This invention pertains to the general field of optical spectrum analyzers, especially as used in communication networks. In particular, the invention relates to an optical spectrum analyzer with a novel implementation of the continuously oscillating scanning mechanism.
2. Description of the Prior Art
Optical spectrum analyzers (often referred to in the art as OSAs) are usually implemented using multilayer-coating filters and diffraction gratings. As illustrated schematically in FIG. 1, in conventional diffraction grating applications a portion of the input beam I is combined with a diffraction grating 10 to separate different wavelengths and direct them in respective separate directions. A mirror 12 is used to reflect a portion of the diffracted spectrum back toward a light detector 14 through collection optics 16,18 suitable for the particular application. (A beam splitter 16 is illustrated, but it is understood that more efficient optics would generally be used in practice.)
When the angular position a of the mirror 12 with respect to the grating 10 is changed (or vice versa), as illustrated by arrow A, the wavelength reflected by the mirror varies. Therefore, the entire spectrum produced by the grating 10 may be collected by the detector 14 by varying the angle of incidence on the mirror 12 through a scan sufficiently wide to cover the spectrum. Typically, this scanning operation is carried out by mounting the mirror on a plate 20 that is alternately rotated by a suitable mechanism 22 in opposite directions over a predetermined angular range −ηmax to +θmax (that is, the mirror is oscillated back and forth). Alternatively, the grating is oscillated in similar manner instead of the mirror.
This conventional approach suffers from several undesirable drawbacks. Because the plate supporting the mirror 12 necessarily changes direction between scan oscillations, the requirements for its mechanical implementation are rigorous, expensive, and often unreliable. The alternating motion of the mirror also limits the speed at which it can be oscillated. Furthermore, a wavelength reference device 24 has to be built into the optical spectrum analyzer in order to synchronize the timing of wavelength reflection by the mirror with the detector reading. That is, each acquisition frame of the detector 14 must be related to a wavelength, which in turn corresponds to an angular position of the mirror 12 in relation to the grating 10. The need for this additional hardware is undesirable because of its expense and potential operating complications.
Similar problems belie optical spectrum analyzers implemented with Michelson interferometers and tunable Fabry-Perot optical filters because they also require rapid oscillating motion. Therefore, the approach of the prior art to optical spectrum analyzer implementation is not particularly efficient for telecommunication applications where speed and miniaturization are essential. Less expensive and more precise technology would be very desirable in the art.
U.S. Pat. No. 7,961,316 provides a novel approach to optical spectrum analyzers based on the idea of varying the angle between the device producing the spectrum and the detector or reflecting mirror through continuous rotation, rather than through oscillating motion. This approach enables the optical spectrum analyzer to function without oscillation of any component and with greater speeds of data acquisition.
In practice, a filter is mounted on a brushless DC (BLDC) motor with an encoder attached to the axis of rotation to provide the angular position of the filter. To reduce the wear of the motor bearing, it is preferred to use the motor in a continuous motion. However, the useful angular range of the filter is only about 20 degrees. Thus, during the 360 degree rotation of the motor, the filter is outside the useful angular range most of the time. As a result, most of the time is wasted for the purposes of the device, and the refresh rate is slow. In addition, the motor and related devices are larger and bulkier than desirable for telecommunication applications. The present invention is directed at providing a simple solution to both problems.